changes failure on request.session.get('LTI_LAUNCH', None) returning None from...

changes failure on request.session.get('LTI_LAUNCH', None) returning None from an AttributeError to an ImproperlyConfigured exception

django_auth_lti/decorators.py

 from functools import wraps
-from django.core.exceptions import PermissionDenied
+from django.core.exceptions import PermissionDenied, ImproperlyConfigured
 from django.utils.decorators import available_attrs
 from django.shortcuts import redirect
 from django.core.urlresolvers import reverse_lazy
@@ -15,6 +15,10 @@ def lti_role_required(allowed_roles, redirect_url=reverse_lazy('not_authorized')
                 allowed = allowed_roles
 
             lti_params = request.session.get('LTI_LAUNCH', None)
+            if lti_params is None:
+                # If this is raised, then likely the project doesn't have
+                # the correct settings or is being run outside of an lti context
+                raise ImproperlyConfigured("No LTI_LAUNCH vale found in session")
             user_roles = lti_params.get('roles', [])
             if set(allowed) & set(user_roles):
                 return view_func(request, *args, **kwargs)